In 1.26 we discovered that several extensions call the 'vscode.openFolder' command with invalid file URIs:

matijarmk.dotnet-core-commands/out/commands/SDKCommands/newproject.js
37,65:                                 vscode.commands.executeCommand("vscode.openFolder", 

When creating a URI from a file path, always use vscode.URI.file(path), e.g vscode.URI.file('c:\\test')
Use vscode.URI.parse(uriString) when the input string is in the form scheme://authority/path

See microsoft/vscode#57267 for more information or for questions.